1. Field of the Invention
The present invention relates to a semiconductor device, in particular a semiconductor device having a level shift circuit and a voltage output circuit.
2. Description of Related Art
Conventionally, a driver circuit of a display device has been composed of a level shift circuit to convert a low amplitude level signal to a high amplitude level signal, and a voltage output circuit to apply high voltage to the display device based on the output of the level shift circuit. For example, Japanese Unexamined Patent Application Publication No. 11-225054 (Isobe et al.) describes a voltage output circuit in related art.
FIGS. 5 and 6 show a driver circuit described in Isobe et al. In FIG. 5, the voltage output circuit is composed of a CMOS type circuit, which is in turn composed of a P-channel type field effect transistor (called “PMOS transistor” hereinafter) and a N-channel type field effect transistor (called “NMOS transistor” hereinafter). Meanwhile, in FIG. 6, the voltage output circuit is composed of two NMOS transistors. In both cases, a voltage output circuit in the related art has circuit structure in which a single transistor is driven by one level shift circuit.
However, in the circuit shown in FIG. 5, the current load characteristic of the transistor in the voltage output circuit largely depends on the power supply potential. Therefore, the lower the power supply potential, the larger the current load characteristic becomes (see FIG. 8). Meanwhile, in the circuit shown in FIG. 6, owing to a zener diode Di inserted between the gate and source of the transistor of the voltage output circuit, the output potential is lower than the power supply potential VCC by an amount corresponding to the voltage drop VZ of the zener diode Di (see FIG. 7).
As explained above, in the case a PMOS transistor is used in the voltage output circuit of the driving circuit in the related art, the current load characteristic depends on the power supply potential, and therefore the lower the power supply potential, the larger the current load resistance becomes. Furthermore, in the case a NMOS transistor is used in the voltage output circuit, the output potential becomes lower than the power supply potential VCC.